Martin Pomerantz

We are studying 2,2'-bithiophenes and 2,2’:5’,2”-terthiophenes with various substituents on the 3-, 3’- and 3”- positions to examine factors involved in producing completely, or nearly completely, planar systems. We are examining properties such as thiophene-thiophene dihedral angles and barriers to rotation, along with any interactions of substituents with the sulfur atoms in adjacent rings, by both theoretical calculations and single crystal X-ray crystallography, after synthesis of these molecules. We have shown theoretically, using ab-initio quantum mechanical calculations, that certain functional groups will stabilize planar (but somewhat distorted) or nearly planar structures because of Coulombic stabilization of polar resonance forms. This stabilization causes some distortion of the molecules, reduced dihedral angles and low rotation barriers. We have also demonstrated theoretically that certain other substituents allow the bithiophene to be completely planar, even with head-to-head substitution. Synthetic routes to these distorted but planar bithiophenes and, therefore, to completely planar, conjugated polythiophenes are being examined. Thus, the carbonyl oxygen has been shown to have a considerable intramolecular Coulombic attraction to a thiophene sulfur atom. When the oxygen atom is replaced by an extremely electronegative fluorine atom, there seems to be essentially no intramolecular attraction between the fluorine and sulfur atoms. However, there is now an intermolecular attraction between these atoms and this considerably alters the morphology of the crystal structure. We are studying the scope of these interactions, which can give rise to self-assembly, in other thiophene and benzothiophene systems as well as in sulfur containing heterocyclic systems where the sulfur has considerable positive charge character, similar to the thiophenes